Related for Ketubot 2:3
הָעֵדִים שֶׁאָמְרוּ כְּתַב יָדֵינוּ הוּא זֶה, אֲבָל אֲנוּסִים הָיִינוּ, קְטַנִּים הָיִינוּ, פְּסוּלֵי עֵדוּת הָיִינוּ, הֲרֵי אֵלּוּ נֶאֱמָנִים. וְאִם יֵשׁ עֵדִים שֶׁהוּא כְתַב יָדָם אוֹ שֶׁהָיָה כְתַב יָדָם יוֹצֵא מִמָּקוֹם אַחֵר, אֵינָן נֶאֱמָנִין:
If witnesses said: This is our signature, but we were forced (to sign) [their lives being threatened (but if it were "money forcing," their money being "threatened" and, it goes without saying, if they said: (We signed) because of the great sum of money given us, that they are not believed; for "a man does not render himself an evildoer." For "a man is kin to himself" and is not believed to testify vis-à-vis himself neither for nor against)], or (if they said) we were minors, or we were unfit to testify, they are believed. [Here, too, (they are believed only if they claim that they were) unfit to testify because of kinship; but if because of transgression, they are not believed.] And if there are witnesses that it is their signature or if their signature were corroborated from a different source, they are not believed. [If their signature appeared on a different deed which had been certified in beth-din, and that deed came before us together with this one, and the signatures were alike (they are not believed)].